Really unique and low system requirments. Definitely best to use a good mixing engineer to get the most out of it. Pairing it with FORTI and SERTI convolution (add on to the Viennna Suite) can really make it sing.

The original S.I.R. is still free, though high latency. TL Space may still be bundled with Pro Tools and Perfect Space is bunled with Sonar. All the others I have encountered cost extra.

Many manufacturers bundle large collections with their engines, like Waves and Audio Ease. One of the only companies that sells collections of real spaces not tied to a given engine is www.NumericalSound.com, though their last library is different and only works with Vienna Audio Suite.